Electric filter



Feb. 1, 1938. H. w. AUGUSTADT 2,105,735

ELECTRIC FILTER Filed May 23, 1936 FIG.

INSERT/0N LOSS 0B N u Q Q Q l l 400 600 800 I000 FREQUENCY-CYCLES PERSECOND INVENTOR H. W AUGUSTADT ATTORNEY Patented Feb. 1, 1938 UNITEDSTATES ELECTRIC FILTER Herbert W. Augustadt, Valley Stream, N. Y.,asslgnor to Bell Telephone Laboratories, Incorporated, New York, NewYork Application May 23,

7 Claims.

This invention relates to electrical filters and more particularly tofilters for smoothing out fluctuations of a direct current.

An object of the invention is to reduce the cost and simplify theconstruction of current supply filters for amplifiers and other spacedischarge devices designed for energization from alternating currentsystems. Another object is to provide an enhanced degree of suppressionof ripple currents in particular ranges of frequencies in the output ofan alternating current rectifier. A further object is the provision ofpower supply circuits of aperiodic character which will be free fromtroublesome transients during operation.

An important application of the invention is found in current supplysystems for vacuum tube apparatus, the energizing currents and voltagesfor which are obtained by rectification of alternating current from apower distribution network. The rectified current in such systemscontains various harmonically related alternating components, the mostimportant of which lie in the frequency range from 120 to 1000 cyclesper second. To provide adequate suppression of the low frequency rippleswith filters of the type heretofore commonly used, it has been necessaryto employ choke coils of very large inductance and usually havingwindings of rather high resistance. Not only are such coils costly tomanufacture and uneconomical of space and weight, but they also absorb aconsiderable portion of the rectified 'voltage and, with theirassociated condensers, tend to give rise to harmful voltage transients.

These difficulties are avoided in the filters of the invention by theuse of a selective network comprising only resistance and capacityelements. The circuit configuration of the network is such as to permitthe proportioning of the elements for the substantially completesuppression of currents of any selected frequency and, at the same time,to provide a large degree of attenuation over a relatively wide adjacentfrequency range. By appropriate choice of the frequency atwhich completesuppression occurs, adequate attenuation of all of the noise producingripples of a rectified current can be secured with a relatively smallloss of the rectified voltage. If desired, the frequency of completesuppression may be made the same as that of the most troublesome ripplecomponent, but experience has shown that this is not necessary althoughit is generally desirable to keep this frequency in the range from about150 to 350 cycles per second when the power is obtained from a 60 cyclesystem.

N. Y., a corporation of 1936, Serial No. s1,4oo

The nature of the invention will be more fully understood from thefollowing detailed description and from the accompanying drawing, ofwhich:

Fig. 1 shows a filter network in accordance with Fig. 3 illustrates theperformance of the filters 10 of the invention.

Referring to Fig. 1, the filter network comprises the circuits includedbetween the terminals t1 t2 and is t4. It consists of two T-networks,one

comprising series resistances R1 and R1 and shunt capacity C1, and theother comprising series capacities C2 and C2 and shunt resistance R2.The two networks are connected to common input terminals t1 and t2 andto common output terminals t3 and t4 thereby providing a pair ofparallel transmission paths. Network R1R1C1 is a filter of the low-passtype which transmits direct currents and low frequency alternatingcurrents with relatively small loss and attenuates high frequencycurrents. filter which provides a high attenuation to low frequencycurrents. The two, in combination, provide a band elimination filterwhich may be proportioned to substantially suppress the transmission ofalternating currents in any selected frequency range and to effect thecomplete suppression of a selected frequency in that range.

In the figure, the filter is shown connected in the power supplycircuits of an audio-frequency amplifier, the plate current for which isobtained by the rectification of an alternating current.

The amplifier and the rectifier are of known types and will be describedonly briefly. The rectifier system comprises a full-wave rectifier I0 ofthe vacuum tube or gas discharge type coupled through supply transformerH to power input terminals l2 and I3. The transformer includesadditional secondary windings for cathode heating purposes in accordancewith common practice. push-pull amplification with resistance-capacitycoupling and is provided with stabilizing feedback circuits inaccordance with the principles described by H. S. Black in an articleentitled Stabilized feed-back amplifiers, Bell System 50 TechnicalJournal, vol. XIII, No. 1, January 1934.

The first stage of the amplifier comprises two suppressor grid pentodetubes l4 and M, the signal input to which is supplied throughtransformer l5. Coupling to the second stage is ef- 5 Network C2C2R2 isa high-pass 25 The amplifier comprises two stages of ing feedback istransmitted from the plates of the second stage tubes throughresistances 2i and 2i to cathode resistors 22 and 22' in the inputcircuits of the first stage, each side of the push-pull system beingseparately stabilized.

The energizing potentials for the anodes and screens of the vacuum tubesare taken from a potential divider comprising resistances 23 and 24 andcondenser 25 connected across the output terminals of the supply filter.The anodes and screens of the second stage are supplied with the fullrectified voltage through conductor 26 and the anodes of the first stagewith a reduced potential through lead 21 and resistor 28. The screens ofthe first stage are connected-through lead 29 to the high voltage sideof resistor 24. The steady bias of the grids of the first amplifierstage is furnished by the fall of potential in resistor 30 which carriesthe steady plate current of that stage. A filter'comprising capacities3i and 32 and resistor 33 prevents plate current fiuctuation fromaffecting the steady grid bias. A similar arrangement is provided forpolarizing the grids of the second stage tubes.

The amplifier described above is representative of a type having highgain and stability suitable for use in sound reproducing systems, andthe like, where the highest degree of fidelity is required. In suchsystems, extraneous noises must be avoided, consequently, therequirements on the current supply filters are of the most severecharacter.

The filters of the invention operate to remove the noise producingfluctuations of the energizing current through the balancing of theoutput currents from the two parallel transmission paths provided by thelow-pass and the high-pass component networks. At very low frequenciesmost of the current is transmitted through the lowpass network RiRi'Ciand such part as traverses the high-pass network CzCz'R: arrives at theoutput terminals in reversed phase. As the frequency increases, theoutput currents tend to become nearly equal in magnitude withoutsubstantial change in their relative phases, with the result that eachsubstantially neutralizes the other in the output. By giving theelements suitable values, complete neutralization of the output currentsmay be secured at some selected frequency together with a high degree ofsuppression over a wide range including the selected frequency. Usually,the strongest component of the ripples in the rectified current is thatcorresponding to the second harmonic of the supply frequency, but it mayfrequently be the case that certain of the higher harmonic componentsare about equally effective in producing noise because of greatersensitivity of the car at these frequencies. I have found that the mosteffective noise reduction is achieved by selecting the frequency ofcomplete suppression some where in the range from about 150 to 350cycles per second.

The design relationships of the filter elements to effect suppression ofa given frequency may be developed as follows:

It will be assumed that the network is of symmetrical configuration suchthat R1 and R1 are equal and also C: and C2. While other relationshipsmay be given, these do not materially alter the properties of the filterand the symmetrical arrangement is one that will usually be preferred inpractice. Under this condition, the symmetrical lattice equivalent tothe network is readily obtained by standard methods. This is illustratedin Fig. 2 in which, for simplicity, only one of each of the equal pairsof branches is shown. The line branch impedances each consist of aparallel connection of resistance R1 and capacity C: and the latticebranches each comprise two parallel impedances, one consisting ofresistance R1 in series with capacity /2C1 and the other consisting ofresistance 2R: in series with capacity C2. The condition for completesuppression of the output current is that the admittance of the linebranches of the equivalent lattice should be equal to the admittance ofthe lattice branches, in which case the circuit becomes a balancedbridge.

In terms of the element values, the foregoing condition is expressed bythe equation:

imaginary parts. Upon simplification, these conditions are found to be:

(2) and where (0 denotes 21 times frequency.

If the frequency of complete suppression be assigned, there remain fourvariables, R1 C1 R2. and C: with only two equations relating to them.This leaves a considerable latitude in the design and permits variationsto meet additional practical requirements. For example, since the seriesresistances determine the amount of the rectified voltage absorbed inthe filter, their value may be chosen with respect to the direct currentresistance of the load so as to limit the voltage loss to apredetermined value. In addition, one or the other of the capacities C1and C: may be fixed arbitrarihr at some convenient value available incommercial condensers or the resistance R: may be assigned in accordancewith a desired high frequency attenuation. In the latter connection itis to be observed that when the frequency is high enough to make theimpedances of the several capacities negligibly small, the filterbecomes equivalent to a simple resistance shunt of magnitude equal tothe resistance of R1 R1 and R: all in parallel. With the seriesresistances fixed from other considerations, the high frequencyattenuation may be controlled by adjusting the value of the shuntresistance R2.

In an amplifier system corresponding to that illustrated in Fig, 1, theoutput load on the filter had an impedance of approximately 12,000 ohmsand the internal impedance of the rectifier was approximately 300 ohms.A filter suitable for use between these impedances has the followingconstants:

R1=250 ohms 01:2 microfarads R2=7.8 ohms 01:16 microfarads The insertionloss characteristic for this filter is shown by curve a of Fig. 3. Theattenuation peak occurs at 100 cycles per second and the attenuation iswell above 45 decibels over the frequency range from 100 to 1000 cyclesper second. The total series resistance or 500 ohms is less than thedirect current resistance of choke coils commonly used in rectifierfilters. The sixteen microiarad capacity oi the condenser C: is wellwithin the range of small sized electrolytic condensers. The losscharacteristic of an alternative filter having an attenuation peak at325 cycles per second is illustrated by curve b of Fig. 3. The constantsfor this filter were:

R1=100 ohms Ci=4 microfarads Ra=8 ohms Ca=12 microfarads As thefrequency of complete suppression is increased, the value of the seriesresistances may be reduced with a consequent reduction of the voltageloss in the filter. Alternatively. the capacity of condensers C: and C1may be diminished or the condenser capacity and resistance value may bereduced together. Some sacrifice of attenuation at the lower frequenciesresults, but this is not serious so long as the suppression frequencydoes not exceed about 350 cycles per second. It is generally desirableto have the shunt resistance Ra of the high-pass section quite low inorder that the attenuation level may be kept high as the frequencyincreases. I have found values in the range from 5 to ohms to besuitable for this purpose. with values between these limits, theattenuation falls of! very slowly above the suppression range and mayeasily be maintained at a level of 40 decibels or greater throughout thewhole audio-frequency range.

40 What is claimed is:

1. An electrical filter comprising a pair of symmetrical T-networksconnected in separate paths between a pair of common input terminals anda pair of common output terminals, one of as said networks consisting 0!two series resistances and a shunt capacity, and the other of saidnetworks consisting of two series capacities and a shunt resistance,said capacities and resistances being proportioned to provide maximumattenugo ation in the filter at a preassigned frequency.

2. An electrical wave filter comprising two T- networks connected inseparate paths between a pair of common input terminals and a pair ofcommon output terminals, one of said networks ll having series branchescontaining only substantially pure resistances and a shunt branchcontaining only capacitive impedance, and the other of said networkshaving series branches containing only capacitive impedance and a shuntbranch consisting of a resistive impedance. said capacities andresistances being proportioned to provide maximum attenuation in thefilter at a preassigned frequency.

3. In combination, a rectifier for alternating currents, a loadimpedance, and an electric filter network connected between the outputterminals of said rectifier and the terminals of said impedance, saidnetwork comprising two parallel transmission paths, a low-pass wavefilter in one of said paths, and a high-pass wave filter in the other ofsaid paths, said low-pass and high-pass filters being proportionedrelatively to each other whereby their output currents neutralize eachother at a frequency in the range between the frequencies of the secondand sixth harmonics of the alternating current supplied to saidrectifier.

4. A system in accordance with claim 3 in which the said low-pass andhigh-pass filters contain resistance elements and reactance elements ofonly one kind.

5. In combination, an alternating current rectifier, a load, and anelectric filter network connected between the output terminals 01' saidrectifier and the terminals of said load, said network comprising twoparallel transmission paths. a low-pass filter comprising only capacityand resistance elements connected in one of said paths, and a high-passfilter comprising only capacity and resistance elements connected in theother of said paths, said low-pass and high-pass filters beingproportioned relatively to each other whereby their output currentsneutralize each other at a frequency in the range between the second andsixth harmonics oi the alternating current supplied to said rectifier.

6. A combination, in accordance with claim 5, in which the said low-passfilter is a T-network of two series resistances and a shunt capacity andthe said high-pass filter is a T-network of two series capacities and ashunt resistance.

7. In combination, a source of fluctuating direct current, a load, alow-pass filter comprising only capacity and resistance elementsconnected between said source and said load, a transmission pathparalleling said filter, and a high pass filter comprising only capacityand resistance elements included in said transmission path, thecapacities and resistances of said filters being proportioned to providesubstantial suppression of current fluctuations o! a preassignedfrequency.

T W. AUGUSTADT.

